Tunable filter

ABSTRACT

A method for implementing a truly hitless tunable filter for use in adding and/or dropping channels in a wavelength division multiplexed (WDM) network is disclosed. An exemplary method for filtering an optical signal may include providing a composite optical signal having several wavelengths including a first wavelength, a second wavelength and other wavelengths. The method may include passing the first wavelength through a filter and reflecting the second wavelength and the other wavelengths with the filter. The method may include making the first wavelength available to a drop port, and making the second wavelength and the other wavelengths available to an output port.

This application is a divisional application of U.S. patent applicationSer. No. 11/147,214 filed on Jun. 8, 2005, now U.S. Pat. No. 7,248,758,which is hereby incorporated by reference.

FIELD OF THE INVENTION

Implementations consistent with the principles of the invention relategenerally to filters used in optical networks, and more particularly, tohitless tunable filters configured for use in an optical communicationnetwork.

BACKGROUND OF THE INVENTION

Modern communication networks may carry data using optical signalstransported across optical fibers. One technique for carryingcommunication data on an optical fiber is wavelength divisionmultiplexing (WDM). WDM is a technique that allows multiple opticalsignals to be carried on a single optical fiber. In WDM, optical signalsmay be separated according to wavelength, where each wavelength maycarry a channel of data. For example, WDM may encode ten channels ofdata onto a single optical fiber by encoding each data channel onto oneof ten different wavelengths.

WDM signals may be dropped from a fiber and/or added to a fiber usingdevices such as re-configurable optical add/drop multiplexers (ROADMs).A ROADM is a device that allows one-or-more wavelengths to be removedfrom, added to, or remain untouched on a WDM fiber. As a result, a ROADMmay be used to “drop” an optical signal from the fiber, such as mightoccur when an optical signal is made available to a user device. A ROADMmay also be used to “add” a signal to a WDM fiber, such as might occurwhen an optical signal is placed onto a WDM fiber by a user device. Anuntouched optical signal may pass through a ROADM without beingmanipulated by the ROADM.

ROADMs may employ optical tuners to drop and/or add optical signals to aWDM fiber. Optical tuners may operate by allowing an optical signal witha particular wavelength to pass through the tuner while reflecting otherwavelengths in the WDM data stream. Optical tuners may disturbwavelengths other than the particular wavelength being added and/ordropped When other wavelengths are impacted, the wavelengths arereferred to as being “hit”. Minimizing and/or eliminating hits may helpWDM networks operate more reliably.

SUMMARY OF THE INVENTION

In accordance with an implementation, a method for filtering an opticalsignal is provided. The method may include providing a composite opticalsignal having several wavelengths including a first wavelength, a secondwavelength and other wavelengths. The method may include passing thefirst wavelength through a filter and reflecting the second wavelengthand the other wavelengths with the filter. The method may include makingthe first wavelength available to a drop port, and making the secondwavelength and the other wavelengths available to an output port.

In accordance with another implementation, a device for filtering anoptical signal is provided. The device may include a tunable filterhaving a first arrangement and configured to receive a composite opticalsignal having a first optical wavelength and a second opticalwavelength. The tunable filter may be configured to pass the firstoptical wavelength and reflect the second optical wavelength. The devicemay include a fixed mirror configured to receive the second opticalwavelength and reflect the second optical wavelength back to the tunablefilter so that the tunable filter can reflect the second opticalwavelength to an output port. The device may include a moveable mirrorconfigured to reflect the first optical wavelength to the tunable filterwhen in a first position. The moveable mirror may be configured to passthe first optical wavelength when in a second position.

In accordance with yet another implementation, a tunable filter isprovided. The tunable filter may include an input port to make acomposite optical signal available, where the composite optical signalhas a first wavelength, a second wavelength and a third wavelength. Thetunable filter may include an output port, a moveable mirror, and atunable filter element positioned in a first orientation, where thefirst orientation is configured to pass the first wavelength to themoveable mirror when the moveable mirror is in a first position, andreflect the second wavelength and the third wavelength. The tunablefilter may include a fixed mirror configured to receive opticalwavelengths reflected from the tunable filter element, and reflect thereceived optical wavelengths back to the tunable filter element via areflecting surface so that the tunable filter element can reflect thereflected optical wavelengths to the output port

In accordance with still another implementation, a hitless tunablefilter is provided. The hitless tunable filter may include means formaking a composite optical signal available to a filtering means via aninput port, means for passing a first wavelength through the filteringmeans and for reflecting at least a second wavelength, means forreflecting the at least second wavelength to an output port inconjunction with the filtering means, and means for reflecting the firstwavelength to the output port where a path traversed by the firstwavelength and a path traversed by the at least second wavelengthbetween the input port and the output port have the same length.

In accordance with yet another implementation, a method for providing afirst wavelength to a destination using a truly hitless tunable filteris provided. The method may include providing a composite input signalhaving the first wavelength and at least one other wavelength to thetruly hitless tunable filter and tuning the truly hitless tunable filterto provide the first wavelength to the destination without hitting theat least one other wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an embodiment of the inventionand, together with the description, explain the invention. In thedrawings,

FIG. 1 illustrates an exemplary system that can be configured to operatein accordance with the principles of the invention;

FIG. 2 illustrates an exemplary implementation of a hitless tunablefilter that may be configured to maintain substantially equivalentoptical path lengths between optical signals reflected from a tunablefilter and between an optical signal passing through the tunable filterconsistent with the principles of the invention;

FIG. 3A illustrates a side view of an exemplary implementation of atunable filter that can be configured to operate on optical signalsconsistent with the principles of the invention;

FIGS. 3B and 3C illustrate top views of the implementation of FIG. 3Afor a small tuning angle orientation and for a large tuning angleorientation, respectively;

FIGS. 4A-4D illustrate an exemplary operating sequence for an exemplaryimplementation of a hitless tunable filter consistent with theprinciples of the invention;

FIGS. 5A and 5B illustrate exemplary techniques for implementing opticalcompensation in implementations of hitless tunable filters consistentwith the principles of the invention;

FIG. 6 illustrates a technique for implementing a monitor port in anexemplary implementation of a hitless tunable filter consistent with theprinciples of the invention;

FIG. 7 illustrates an exemplary hitless tunable filter that may employmultiple reflections to increase the extinction ratio of an exemplaryimplementation consistent with the principles of the invention;

FIGS. 8A and 8B illustrate a side view and top view, respectively, of anexemplary implementation of a hitless tunable filter consistent with theprinciples of the invention; and

FIG. 9 illustrates an exemplary device that employs four hitless tunablefilters operating in a cascade arrangement consistent with theprinciples of the invention.

DETAILED DESCRIPTION

The following detailed description of implementations consistent withthe principles of the invention refers to the accompanying drawings. Thesame reference numbers in different drawings may identify the same orsimilar elements. Also, the following detailed description does notlimit the invention. Instead, the scope of the invention is defined bythe appended claims and their equivalents.

Implementations may include a truly hitless tunable filter operatingalone or in combination with other devices, such as additional trulyhitless tunable filters. For example, implementations of truly hitlesstunable filters may be cascaded together and deployed within a devicesuch as a ROADM. Implementations may perform optical switching of adesired wavelength without hitting other wavelengths that may be presenton a WDM fiber. In addition, implementations may provide truly hitlesstunable filters having low insertion losses that may be due, at least inpart, to relatively simple optical paths used within theimplementations.

Exemplary System

FIG. 1 illustrates an exemplary system that can be configured to operatein accordance with the principles of the invention. System 100 mayinclude a network 102, a first user 104-1, a second user 104-2, a firstROADM 106-1, a second ROADM 106-2, and a service provider 108.

Network 102 may include any network capable of carrying optical datausing one or more optical fibers. In one implementation, network 102 maybe a wave division multiplexed (WDM) network, such as a dense WDM (DWDM)or a coarse WDM (CWDM). Network 102 may be a local area network (LAN),such as a network associated with a university campus, a metropolitanarea network (MAN), such as a city wide network, and/or a wide areanetwork (WAN), such as an Internet network. Network 102 may supportsubstantially any networking protocol, such as asynchronous transfermode (ATM), Internet protocol (IP), synchronous optical transport(SONET), and/or transmission control protocol (TCP). Network 102 maycarry multiple optical wavelengths on a single fiber, where eachwavelength may be associated with a data channel carried via thefiber(s). For example, a first channel may be used by first user 104-1and encoded via a first wavelength and a second channel may be used bysecond user 104-2 and encoded via a second wavelength.

First user 104-1 and/or second user 104-2 (also referred to as user 104)may include any device and/or system configured to accept data fromand/or place data on network 102. For example, user 104 may include aLAN associated with a corporation. The LAN may include one or moredevices, such as servers, routers, switches, firewalls, and/or networkaddress translators (NATs). User 104 may interact with network 102 viaone-or-more optical channels configured to carry data to/from network102.

First ROADM 106-1 and/or second ROADM 106-2 (also referred to as ROADM106) may include any device capable of dropping a DWDM channel, adding aDWDM channel and/or passing a DWDM channel from an input port to anoutput port. For example, ROADM 106-1 may be configured to add a DWDMchannel to network 102, such as might occur if first user 104-1 attemptsto place data on network 102. ROADM 106-1 may also be configured to dropa channel from network 102, such as might occur if first user 104-1receives data from network 102. ROADM 106-1 may also be configured topass data without adding or dropping a channel, such as might occur ifdata is sent from service provider 108 to ROADM 106-2 in a clockwisedirection passing through ROADM 106-1.

Service provider 108 may include any device configured to operate withnetwork 102. For example, service provider 108 may be configured toplace data onto network 102, remove data from network 102, and/orcontrol devices and/or data associated with network 102, such as ROADM106-1 and/or ROADM 106-2. Service provider 108 may operate inconjunction with a dedicated ROADM 106 and/or may be configured tooperate directly with network 102 by incorporating ROADM-likefunctionality within devices and/or systems associated with serviceprovider 108.

Implementations of system 100 may support substantially any number ofDWDM channels using one-or-more optical fibers. System 100 may alsosupport substantially any number of users, devices, and/or serviceproviders without departing from the spirit of the invention.

Exemplary Filter Implementation

FIG. 2 illustrates an exemplary implementation of a hitless tunablefilter that may be configured to maintain substantially equivalentoptical path lengths between optical signals reflected from a tunablefilter and between an optical signal passing through the tunable filterconsistent with the principles of the invention. In one implementation,the hitless tunable filter is included with a ROADM 106. In anotherimplementation, the hitless tunable filter may be separate from ROADM106, such as within service provider 108.

The implementation of FIG. 2 may depict a general representation andarrangement of components that can be used for implementing aspects ofthe invention. The implementation of FIG. 2 may employ components havingconfigurations such as curved mirrors, flat mirrors,micro-electromechanical switch (MEMS) mirrors, thin film tunablefilters, thermally tuned filters, MEMS tunable filters, and/or othercomponents for manipulating optical signals. Implementations employingcertain of these components in selected configurations maybe describedin conjunction with subsequent figures.

As shown in FIG. 2, the hitless tunable filter may include an input port202, an output port 204, an add port 206, a drop port 208, a tunablefilter 210, a fixed mirror 212, and a moveable mirror 214. Input port202, may include any device configured to make a composite opticalsignal available to tunable filter 210 and/or moveable mirror 214. Forexample, a composite input signal may include a group of opticalwavelengths. Input port 202 may include a collimating lens for focusingincoming wavelengths on a determined location associated with a surfaceof tunable filter 210. Output port 204 may include any device configuredto receive one or more optical wavelengths from tunable filter 210and/or to make the one or more optical wavelengths available to anotherfiber and/or device.

Add port 206 may include any device configured to make one or moreoptical wavelengths available to moveable mirror 214, tunable filter 210and/or output port 204. In one implementation, add port 206 may beconfigured in a manner substantially similar to input port 202. Dropport 208 may include any device configured to make one or more opticalwavelengths available to another optical fiber and/or device. In oneimplementation, drop port 208 may be configured in a mannersubstantially similar to output port 204.

Tunable filter 210 may include any device capable of receiving one ormore optical wavelengths at a first surface and reflecting one or moreoptical wavelengths to another device, such as fixed mirror 212, and/orpassing one or more optical wavelengths from a first surface to a secondsurface en route to another device, such as moveable mirror 214 and/ordrop port 208. Implementations of tunable filter 210 may pass and/orreflect particular optical wavelengths as a function of the incidentangle of the wavelengths, as a function of the temperature of tunablefilter 210, and/or as a function of the position of tunable filter 210.Implementations of tunable filter 210 may be configured to receiveoptical signals via free space.

Fixed mirror 212 may include any device configured to reflect opticalsignals to a destination. Fixed mirror 212 may be configured to reflectoptical signals back to a location from which the optical signals werereceived and/or to reflect optical signals to another location. Fixedmirror 212 may operate alone or in conjunction with other devices, suchas lenses, prisms, and/or other optical and/or electro-optical elements.

Moveable mirror 214 may include any device configured to reflect opticalsignals to another device and/or location, such as tunable filter 210and/or output port 204, via a reflecting surface. Implementations mayinclude a moveable mirror 214 configured to pass optical signalsreceived at a first surface to another device and/or location via asecond surface. Moveable mirror 214 may have two primary positions, suchas a tuning position and/or a working position.

In a tuning position, moveable mirror 214 may be positioned as shown inFIG. 2 so that optical signals incident on moveable mirror 214 may bereflected to a destination, such as output port 204. In a workingposition, moveable mirror 214 may be positioned so that one or morewavelengths pass through tunable filter 210 and reach drop port 208 viaa path, such as C1, without contacting moveable mirror 214. Whenmoveable mirror 214 is in a working position, optical signals from addport 206 may reach output port 204 since moveable mirror 214 may not belocated in the optical path between add port 206 and output port 204.

The implementation of FIG. 2 may operate as a truly hitless tunablefilter with a composite optical signal containing, for example,wavelengths of λ1, λ2, λ3, λ4, and λ5. The implementation of FIG. 2 mayoperate by switching between one wavelength, such as λ1, and anotherwavelength, such as λ4, without hitting any intervening wavelengths,such as λ2 and/or λ3. λ2 and λ3 are hit if they are disturbed whenswitching from, for example, λ1 to λ4. Implementations may operatewithout hitting wavelengths other than the wavelengths being switched.

By way of example, assume that the composite optical signal is madeavailable to tunable filter 210 via input port 202. The composite signalmay traverse path A1 between input port 202 and tunable filter 210.Wavelengths λ2-λ5 may be reflected from tunable filter 210 to fixedmirror 212 via path A2 and from fixed mirror 212 back to tunable filter210 via path A3. Tunable filter 210 maybe configured to reflectwavelengths λ2-λ5 to output port 204 via path A4. Tunable filter 210 maybe positioned so that λ1 passes from a first surface of tunable filter210 to a second surface of tunable filter 210 while wavelengths λ2-λ5are reflected via the first surface of tunable filter 210. Path B1 maybe traversed by λ1 en route to a reflecting surface of moveable mirror214. The implementation of FIG. 2 may be configured and arranged suchthat path B1 is one-half the length of path A1+A2+A3+A4.

In a working mode, moveable mirror 214 may be positioned so as not to bein a path taken by λ1 and/or other wavelengths passing through tunablefilter 210, so that λ1 may be made available to drop port 208.Alternatively, a wavelength, such as λ6, may be made available to outputport 204 via add port 206 when moveable mirror 214 is positioned inaccordance with a working mode. A wavelength, such as λ6, may traversepath D1+B2 en route to output port 204. The implementation of FIG. 2 maybe configured such that path D1 is the same length as path C1 and/or B1.

A tuning mode may be employed to switch from one dropped wavelength,such as λ1, to another wavelength, such as λ4. In a tuning mode,moveable mirror 214 may be configured to reflect λ1 to output port 204via path B2. As a result, wavelengths λ1-λ5 may be present at outputport 204 in the tuning mode. If path B2 is configured to be the samelength as path B1,B1+B2=A1+A2+A3+A4  (Eq. 1)

Equation 1 indicates that the tuning mode optical path length from inputport 202 to output port 204 may be the same for optical signalsreflected from tunable filter 210 (e.g., wavelengths λ2-λ5) as it is forsignals passing through tunable filter 210 and being reflected frommoveable mirror 214 (e.g., λ1). When the A1-A4 and B1-B2 optical pathlengths are the same, signals at output port 204 may appear to havetraveled the same distance regardless of the particular A or B pathtraversed. Downstream devices on network 102 may not detect any relativedifferences that are attributable to path length delays between λ1 andwavelengths λ2-λ5.

Tunable filter 210 may be positioned, or repositioned, to allow otherwavelengths to reach moveable mirror 214. For example, tunable filter210 may be repositioned with respect to wavelengths λ1-λ5 so as to allowonly λ4 to reach moveable mirror 214. Moveable mirror 214 may be placedin a working mode position so that λ4 is made available to drop port208.

The implementation of FIG. 2 may allow determined wavelengths toselectively pass through tunable filter 210 without hitting otherwavelengths. For example, λ2 and λ3 may not be disturbed during thetuning operation when tunable filter 210 is tuned from λ1 to λ4. Assuch, the implementation of FIG. 2, as well as other implementationsdescribed herein, may operate as truly hitless tunable filters. Theimplementation of FIG. 2 may be configured so that path lengths C1 andD1 are substantially the same length as paths B1 and B2.

Exemplary Hitless Filter Configuration

FIG. 3A illustrates a side view of an exemplary implementation of atunable filter that can be configured to operate on optical signalsconsistent with the principles of the invention. The implementation ofFIG. 3A may include input port 202, output port 204, add port 206, dropport 208, tunable filter 310, fixed mirror 312, and/or moveable mirror314. Tunable filter 310 may be a thin film tunable filter that may passone or more wavelengths as a function of the angle formed by incidentoptical signals on the first surface of tunable filter 310. A thin filmimplementation may be configured and adapted with a surface coating thatallows tunable filter 310 to pass a single wavelength or more than onewavelength as a function of an incident angle. Tunable filter 310 may beselected according to a working range of wavelengths that will be usedfor a particular application, such as in a particular implementation ofnetwork 102.

Fixed mirror 312 may include a mirror employing a curved reflectingsurface facing tunable filter 310. The curved surface may be adapted toreflect an incident optical signal back to a location on tunable filter310 that may correspond to the location from which the incident opticalsignal originated on the first surface of tunable filter 310. In oneimplementation, fixed mirror 312 may be configured so that wavelengthsassociated with an incident beam strike a center of curvature associatedwith fixed mirror 312. Moveable mirror 314 may include a mirror having asubstantially flat reflecting surface and/or a curved reflectingsurface.

FIGS. 3B and 3C illustrate top views of the implementation of FIG. 3Afor a small tuning angle orientation and for a large tuning angleorientation, respectively. In FIGS. 3B and 3C input port 202 may liedirectly over output port 204 and add port 206 may lie directly overdrop port 208 as viewed on a page. As a result, only two of the fourports may be visible in these figures. In other implementations, thefour ports may be oriented differently.

Angle 316 (FIG. 3B) may represent a small tuning angle geometry. Angle316 may be associated with a particular wavelength, such as λ1 in theexample discussed in conjunction with FIG. 2. In contrast, angle 318(FIG. 3C) may be associated with a large tuning angle geometry. Angle318 may be associated with, for example, λ4 as discussed in conjunctionwith FIG. 2. Implementations may be adapted to operate oversubstantially any range of tuning angles depending on the wavelengthsused, geometry of components used to filter particular wavelengths,coatings used on tunable filter 310, curvatures used on fixed mirror312, and types of mirrors used for moveable mirror 314. Therefore,implementations are not limited to any particular range of workingand/or tuning angles.

Exemplary Sequence of Operation

FIGS. 4A-4D illustrate an exemplary operating sequence for an exemplaryimplementation of a hitless tunable filter consistent with theprinciples of the invention. FIGS. 4A-4D may include input port 202,output port 204, add port 206, drop port 208, tunable filter 310, fixedmirror 312, and moveable mirror 314.

FIG. 4A illustrates a working mode for a first wavelength, such as λ1.In the working mode, λ1 may be made available to drop port 208 afterpassing through tunable filter 310. FIG. 4B illustrates a tuning modefor the first wavelength. In the tuning mode, the first wavelength maybe reflected by moveable mirror 314 so that all wavelengths incident ontunable filter 310 may be made available to an output port, such asoutput port 204.

FIG. 4C illustrates the tuning mode of FIG. 4B where tunable filter 410may be manipulated to another wavelength, such as λ4. All wavelengthsincident on tunable filter 310 may be made available to an output port204 in the implementation of FIG. 4C. FIG. 4D illustrates a working modewhere a new wavelength may be made available to drop port 208. Theoperational sequence of FIGS. 4A through 4D may tune from one wavelengthto another wavelength without hitting wavelengths lying between theinitial wavelength (FIG. 4A) and the later tuned wavelength (FIG. 4D).For example, in the implementation of FIGS. 4A-4D, λ2, λ3 and λ5 may notbe hit when switching from λ1 to λ4.

Exemplary Compensation Techniques

FIGS. 5A and 5B illustrate exemplary techniques for implementing opticalcompensation in implementations of hitless tunable filters consistentwith the principles of the invention. Implementations may be adapted tocompensate for focusing and/or path length aspects associated withcomponents such as input port 202, tunable filter 310 and/or fixedmirror 312. For example, in FIG. 5A an optical piece 502 may beconfigured and adapted to compensate for focus and/or optical pathlength. Optical piece 502 may include any device configured to induce avariation in an optical path associated with an optical beam. Opticalpiece 502 may include an optical component, such as a prism, piece ofglass, a lens and/or an opto-electrical component that may changeoptical characteristics as a function of an applied electrical potentialand/or current.

FIG. 5B illustrates the implementation of FIG. 5A along with acylindrical lens 504. Input port 202 may provide a focused compositebeam via a collimating lens. The implementation of FIG. 5B may operateefficiently when reflections from fixed mirror 312 are focused ontunable filter 310 with a desired resolution. Receiving composite beamsvia focusing devices may cause the focusing contribution of fixed mirror312 to exceed a desired resolution. The implementation of FIG. 5B mayemploy focusing and/or defocusing devices, such as cylindrical lens 504,to correct for focusing/defocusing contributions attributable to othercomponents.

Cylindrical lens 504 may include any device configured to focus and/ordefocus an optical beam. Cylindrical lens 504 may operate on a singlewavelength or may operate on a composite beam. Cylindrical lens 504 maybe configured to compensate one or more wavelengths in one direction,such as in a plane of curvature of fixed mirror 312. By correcting in asingle direction, cylindrical lens 504 may ensure that an output beamincluding one or more wavelengths is substantially circular. Cylindricallens 504 may operate with, or without, optical piece 502 when operatingon one or more wavelengths interacting with tunable filter 310, fixedmirror 312 and/or moveable mirror 314.

Exemplary Monitor Port Implementation

FIG. 6 illustrates a technique for implementing a monitor port in anexemplary implementation of a hitless tunable filter consistent with theprinciples of the invention. The implementation of FIG. 6 may includeinput port 202, output port 204, add port 206, drop port 208, tunablefilter 310, fixed mirror 312, moveable mirror 314, optical piece 502,tap 602 and monitor port 604. Tap 602 may include any device configuredto make one or more wavelengths available to another device. Monitorport 604 may include any device configured to receive an optical and/orelectrical signal associated with a monitored signal. For example, inone implementation, tap 602 may include a photo detector operativelycoupled to a receiving device, such as an analog-to-digital converterOther implementations may use a piece of glass and or other tappingdevices for tap 602.

Exemplary Implementation for Increasing Extinction Ratio

FIG. 7 illustrates an exemplary hitless tunable filter that may employmultiple reflections to increase the extinction ratio of an exemplaryimplementation consistent with the principles of the invention.Extinction ratio may refer to a ratio of the power associated with anoptical representation of a binary “one” to the power associated with anoptical representation of a binary “zero”. Implementations may improvethe extinction ratio by employing additional reflected paths between atunable filter and a fixed mirror.

FIG. 7 illustrates one such implementation that may be used to improvean extinction ratio and may include input port 202, output port 204, addport 206, drop port 208, tunable filter 710, fixed mirror 712, andmoveable mirror 714. Tunable filter 710 may be adapted for operationusing multiple reflections and/or may operate in a manner similar totunable mirror 310 and may be adapted to accommodate multiplereflections with fixed mirror 712. For example, the implementation ofFIG. 7 may employ paths A1-A4 as illustrated in FIG. 2 along withadditional reflected paths A5-A7. The additional reflected paths A5-A7may operate to increase the extinction ratio of the implementation ofFIG. 7 as compared to, for example, the implementation of FIG. 2.

Fixed mirror 712 may be configured and adapted to operate with multiplereflections and/or to operate in a manner similar to fixed mirror 312.Fixed mirror 712 may be capable of accommodating additional incidentpaths, such as path A5, and may be capable of accommodating additionalreflected paths, such as path A6. Moveable mirror 714 may be configuredas a flat mirror and/or a curved mirror.

Exemplary MEMS Implementation

FIGS. 8A and 8B illustrate a side view and top view, respectively, of anexemplary implementation of a hitless tunable filter consistent with theprinciples of the invention. The implementations of FIGS. 8A and 8B mayinclude input port 202, output port 204, add port 206, drop port 208,micro-electrical mechanical switch (MEMS) tunable filter 810, flat-fixedmirror 812, and MEMS mirror 814. MEMS tunable mirror 810 may include anyMEMS device configured and adapted to reflect and/or pass one or moreoptical wavelengths. MEMS tunable filter 810 may operate to reflectand/or pass wavelengths as a function of the angle of an incidentwavelength, as a function of the temperature of the tunable filter,and/or as a function of the thickness of the tunable filter. MEMStunable filter 810 may be operated by an electrical and/orelectro-mechanical source to position MEMS tunable filter 810 in adetermined position to filter incoming optical signals. Flat-fixedmirror 812 may include any device configured to reflect an incidentoptical signal using a substantially flat reflecting surface.

MEMS mirror 814 may include any MEMS compatible device for reflectingand/or passing an incident optical signal to a determined location. MEMSmirror 814 may be moveably operated to displace MEMS mirror 814 from atuning position, where an incident optical signal is reflected, to aworking position, where MEMS mirror 814 does not interact with anincoming optical signal. When MEMS mirror 814 is in a tuning position,an incoming optical signal is reflected to output port 204 after passingthrough MEMS tunable filter 810. When MEMS mirror 814 is in a workingposition, an incoming optical signal may pass through MEMS tunablefilter 810 en route to drop port 208.

Exemplary Cascade Implementation

FIG. 9 illustrates an exemplary device that employs four hitless tunablefilters operating in a cascade arrangement consistent with theprinciples of the invention. Device 900 may include tunable filterassemblies 902A-902D, an input port 904, an output port 906, add ports908, and drop ports 910. While four filter assemblies are illustrated inFIG. 9, there may be more or fewer assemblies in other implementations.Tunable assemblies 902A-D may be configured and may operate aspreviously described in conjunction with FIGS. 2, 3A-C, 4A-D, 5A-B, and6. Tunable assemblies 902A-D may also be configured and may operate aspreviously described in conjunction with FIGS. 7, and 8A and 8B.

Tunable assemblies 902A-D may be arranged in a cascade configuration.The cascade configuration may couple an output port from one tunableassembly to an input port associated with a neighboring tunableassembly. For example, an output port of tunable assembly 902A may beoperatively coupled to an input port associated with tunable assembly902B, an output port associated with tunable assembly 902B may beoperatively coupled to an input port associated with tunable assembly902C, and an output port associated with tunable assembly 902C may beoperatively coupled to an input port associated with tunable assembly902D. Device 900 may have one input port 904 and one output port 906accessible for connections to external signals lines, such as opticalfibers.

Device 900 may include an add port and/or a drop port for each tunableassembly 902A-D. For example, the implementation of FIG. 9 may includefour add ports 908 and four drop ports 910. Implementations such asdevice 900 may facilitate the adding and/or dropping of multiplewavelengths using a single integrated device. For example, wavelengthsλ1-λ5 maybe coupled to input port 904. Assembly 902A may drop λ4,assembly 902B may drop λ3, assembly 902C may drop λ2, and assembly 902Dmay drop λ1. Output port 906 may make λ5 available to other devices,such as other devices on network 102. Implementations such as device 900may be adapted to operate in ROADMs and/or other network devices.

CONCLUSION

Implementations consistent with the principles of the inventionfacilitate deployment of truly hitless tunable filters.

The foregoing description of exemplary embodiments of the inventionprovides illustration and description, but is not intended to beexhaustive or to limit the invention to the precise form disclosed.Modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention.

For example, implementations consistent with the principles of theinvention can be implemented using assemblies and parts other than thoseillustrated in the figures and described in the specification withoutdeparting from the spirit of the invention. Parts may be added and/orremoved from the implementations of FIGS. 1-9 depending on specificdeployments and/or applications. Further, disclosed implementations maynot be limited to any specific combination of hardware.

No element, act, or instruction used in the description of the inventionshould be construed as critical or essential to the invention unlessexplicitly described as such. Also, as used herein, the article “a” isintended to include one or more items. Where only one item is intended,the term “one” or similar language is used. Further, the phrase “basedon,” as used herein is intended to mean “based, at least in part, on”unless explicitly stated otherwise.

The scope of the invention is defined by the claims and theirequivalents.

1. A tunable filter comprising: an input port to make a compositeoptical signal available, where the composite optical signal has a firstwavelength, a second wavelength and a third wavelength; an output port;a moveable mirror; a tunable filter element positioned in a firstorientation, the first orientation configured to: pass the firstwavelength to the moveable mirror when the moveable mirror is in a firstposition, and reflect the second wavelength and the third wavelength;and a fixed mirror configured to: receive optical wavelengths reflectedfrom the tunable filter element, and reflect the received opticalwavelengths back to the tunable filter element via a reflecting surfaceso that the tunable filter element can reflect the reflected opticalwavelengths to the output port.
 2. The tunable filter of claim 1,wherein when the tunable filter element is positioned in a secondorientation, the tunable filter element is configured to: pass the thirdwavelength to the moveable mirror, and reflect the first wavelength andthe second wavelength.
 3. The tunable filter of claim 2, wherein thetransition from the first orientation to the second orientation does notcause a collision with the second wavelength.
 4. The tunable filter ofclaim 1, wherein the moveable mirror is moved to permit the firstwavelength to be made available to a drop port.
 5. The tunable filter ofclaim 1, wherein the fixed mirror has a curved reflecting surfaceconfigured to: receive reflected wavelengths at a center of curvature onthe curved reflecting surface.
 6. The tunable filter of claim 1, whereina length of a first path taken by the first wavelength is the same as alength of a second path taken by the second wavelength and the thirdwavelength.
 7. The tunable filter of claim 6, wherein the second pathextends from the input port to a reflecting surface of the tunablefilter element, from the reflecting surface of the tunable filterelement to the reflecting surface of the fixed mirror, from thereflecting surface of the fixed mirror to the reflecting surface of thetunable filter element and from the reflecting surface of the tunablefilter element to the output port, and wherein the first path extendsfrom the input port to a reflecting surface on the moveable mirror andfrom the reflecting surface on the moveable mirror to the output port.8. The tunable filter of claim 1, further comprising: a cylindrical lenspositioned between the input port and the tunable filter element, wherethe cylindrical lens focuses or defocuses an optical signal thatcontains the first wavelength, the second wavelength and the thirdwavelength.
 9. A tunable filter, comprising: means for making acomposite optical signal available to a filtering means via an inputport; means for passing a first wavelength through the filtering meansand for reflecting at least a second wavelength; means for reflectingthe at least second wavelength to an output port in conjunction with thefiltering means; and means for reflecting the first wavelength to theoutput port where a path traversed by the first wavelength and a pathtraversed by the at least a second wavelength between the input port andthe output port have the same length.
 10. A tunable filter, comprising:a tunable filtering component to: pass a first wavelength to a moveablemirror in the tunable filter when the tunable filtering component is ina first position, where the moveable mirror sends the first wavelengthto an output port of the tunable filter via a first path in a firstoperating mode and allows the first wavelength to be sent to a drop portof the tunable filter in a second operating mode, reflect a group ofwavelengths to the output port via a second path when the tunablefiltering component is in the first position, pass a second wavelengthto the moveable mirror when the tunable filtering component is in asecond position, where the second wavelength is one of the wavelengthsin the group of wavelengths, and where the moveable mirror reflects thesecond wavelength to the output port in the first operating mode andallows the second wavelength to be sent to the drop port in the secondoperating mode without hitting the first wavelength or other wavelengthsin the group of wavelengths, and reflect the first wavelength to theoutput port via the second path when the tunable filter component is inthe second position.
 11. The tunable filter of claim 10, wherein themoveable mirror is in a first relationship with respect to the outputport in the first operating mode and a second relationship with respectto the output port in the second operating mode.
 12. The tunable filterof claim 10, wherein the tunable filtering component passes or reflectswavelengths in the group of wavelengths based on a relationship of thetunable filtering component to wavelengths in the group of wavelengths.13. The tunable filter of claim 12, wherein the relationship includes anangular relationship of wavelengths in the group of wavelengths withrespect to a surface of the tunable filtering component.
 14. The tunablefilter of claim 12, wherein the tunable filtering component is a thinfilm filtering component.
 15. The tunable filter of claim 10, furthercomprising a fixed mirror that employs a curved surface to reflectwavelengths in the group of wavelengths toward the output port via thesecond path.
 16. The tunable filter of claim 10, further comprising: anoptical compensation component to optically compensate wavelengths inthe first path or the second path.
 17. The tunable filter of claim 10,further comprising a fixed mirror that is adapted to increase anextinction ratio related to wavelengths on the second path.
 18. Atunable filter, comprising: a first tunable stage to: receive aplurality of wavelengths via an input port, pass a first one of theplurality of wavelengths to a first drop port, reflect others of theplurality of wavelengths to a first output port via a mirror, and tunefrom passing the first one of the plurality of wavelengths to passing asecond one of the plurality of wavelengths without hitting otherwavelengths in the plurality of wavelengths; and a second tunable stageto: receive the others of the plurality of wavelengths from the firstoutput port, pass one of the others of the plurality of wavelengths to asecond drop port, pass remaining ones of the others of the plurality ofwavelengths to a second output port, and tune from passing the one ofthe others of the plurality of wavelengths to passing another one of theothers of the plurality of wavelengths.
 19. The tunable filter of claim18, wherein the first tunable stage or the second tunable stagecomprises: a thin film tunable filter.
 20. The tunable filter of claim18, wherein the first tunable stage comprises: a moveable mirror toreflect the first one of the plurality of wavelengths to the firstoutput port when in a first position and to allow the first one of theplurality of wavelengths to reach the first drop port when in a secondposition.
 21. The tunable filter of claim 18, wherein the first tunablestage comprises: a first mirror to reflect the other of the plurality ofwavelengths to the first output port.
 22. The tunable filter of claim18, wherein the first tunable stage or the second tunable stage are usedin a re-configurable optical add/drop multiplexer (ROADM).